There are known aircraft equipped with engines configurations such as Open Rotor (OR) or Turbofan Boundary Layer Ingestion (BLI). In these aircraft, potential hazardous events occur, such as a Propeller Blade Release (PBR) event, i.e., an event where an external blade of one turboprop engine comes off and hits the fuselage, or an Uncontained Engine Rotor Failure (UERF) event, i.e., an event where a part of the internal rotor of the engine breaks, it is released and hits the fuselage, can generate large damage on the fuselage, and also in the opposite engine.
Although engine manufacturers are making efforts to reduce the probability of such failure events, experience shows that PBR and UERF events that can lead to catastrophic events continue to occur.
In the case of a Propeller Blade Release event no special protections are applied in current aircraft, however, some studies have been performed in order to assess the carbon fiber composite thickness required by the fuselage in the potential impact area to resist such an event.
In terms of protection for UERF events, there are applied some protections in order to minimize the hazards of an engine or an Auxiliary Power Unit (APU) rotor failures. Additionally, a particular protection is applied on fuel tanks if they are located in impact areas, in order to minimize the possibility of fuel cell damage. The shielding of aluminum or titanium is typically used for these events.
As it is well known, weight is a fundamental aspect in the aeronautic industry and therefore there is a trend to use structures of a composite material instead of a metallic material even for primary structures such as fuselages. However, the usual composite materials made of carbon fibers, compared to conventional light weight metallic materials, presents a lower impact resistance and lower damage tolerance capabilities. Also, no plasticity behavior as on metallic materials is present in composite materials and they are not able to absorb high strain energy amounts when deforming.
Depending on the threat, the most widely spread ballistic composite armors are typically composed of layers of different materials, such as metal, fabrics and ceramics or by sole fabrics of materials with good ballistic performance, also called “dry” fabrics.
Thus, there is a need of fuselage structures capable of satisfying the safety requirements and ballistic performance particularly when they are made up of composite materials.